This application claims the priority of Korean Patent Application No. 2003-35563, filed on Jun. 3, 2003, in the Korean Intellectual Property Office, the contents of which are incorporated herein in their entirety by reference.
1. Field of the Invention
The present invention relates to a semiconductor integrated circuit (IC), and more particularly, to a method of removing noise occurring when an analog signal of a semiconductor integrated circuit (IC) is controlled using feedback, and a system therefor.
2. Description of the Related Art
An analog circuit for a semiconductor integrated circuit (IC) feeds back its own output and compares the fed-back output with a level of a reference signal, so as to obtain a desired output level. The analog circuit includes a phase locked loop (PLL) or a delay locked loop (DLL) and an impedance control circuit or an output current control circuit for an output driver.
In order to control the output of the analog circuit, a detector which compares the output of the analog circuit with a reference value, a controller which receives the output of the detector and controls an output value of the analog circuit, and a state-machine which stores and indicates a current state, should be provided.
Feedback control methods include an analog feedback control method and a digital feedback control method. In the analog feedback control method, the output of the detector is an analog level proportional to a difference between a reference value and an output level of the analog circuit, where the state of the state-machine indicates a consecutive value. Thus, an output state also indicates a consecutive value.
On the contrary, in the digital feedback control method, the output of the detector has a finite discrete value, such as 1 or 0, and an output value is obtained by varying the value of a finite state machine (FSM) (mostly, counter) which is a state-machine and controlling the analog circuit based on the value. Thus, in the digital feedback control method, the output value is not consecutive but is quantized according to the size and resolution of the FSM.
Both the analog feedback control method and the digital feedback control method have advantages and disadvantages. However, since due to control using the FSM, the current state is stored and a variety of processing is possible, the digital feedback control method is widely used.
FIG. 1 is a block diagram of a conventional digital feedback control method performed using an analog circuit.
A detector 110 compares a current output value OUT with a reference value REF and determines the variation direction of an output. According to a determination result, the detector 110 outputs a rising signal or a falling signal UP/DOWN, and a counter 130 controls the value of the counter 130 of a finite state machine (FSM) 120 in response to the rising signal or the falling signal UP/DOWN.
The output of the counter 130 is applied to a digital-to-analog converter (DAC) 140 and converted into an analog control signal CTRL, and an output value OUT of an analog circuit, i.e., an output value OUT of a generator 150 is controlled using the converted analog control signal CTRL.
For example, if the current output value OUT is smaller than the desired reference value REF, the detector 110 increases the value of the counter 130 by generating a rising signal UP. As a result, the level of the analog control signal CTRL is increased. Thus, the output value OUT is increased and reaches the reference value REF. By repeating the above operation, the output value OUT is almost the same as the reference value REF.
Coming and going based on the reference value REF from when the output value OUT is almost the same as the reference value REF, is repeated and is referred to as bang—bang. For example, if the output value OUT is a little smaller than the reference value REF, the detector 110 generates a rising signal UP and increases the output value OUT by 1.
However, if the output value OUT increased by 1 is higher than the reference value REF, during a next comparison operation, the detector 110 generates a falling signal DOWN and controls the output value OUT to return to its original state, and the above operation is continuously repeated. The above operation is referred to as a bang—bang operation and is shown in FIG. 2.
The bang—bang operation causes a noise component for an analog output value. In a delay locked loop (DLL) and also causes jitter noise.